Recently, chemical amplification-type DUV (deep ultra violet) photoresists have proven to be useful to achieve high sensitivity in processes for preparing micro-circuits in the manufacture of semiconductors. These photoresists are prepared by blending a photoacid generator with polymer matrix macromolecules having acid labile structures.
According to the reaction mechanism of such a photoresist, the photoacid generator generates acid when it is irradiated by the light source, and the main chain or branched chain of the polymer matrix in the exposed portion is reacted with the generated acid to be decomposed or cross-linked, so that the polarity of the polymer is considerably altered. This alteration of polarity results in a solubility difference in the developing solution between the exposed area and the unexposed area, thereby forming a positive or negative image of a mask on the substrate.
In some photoresists, functional groups on the main chain or branched chain of one polymer are cross-linked with the main chain or branched chain of another polymer in the matrix. A cross-linker is therefore added to the photoresist to promote cross-linking between the polymers.
However, a cross-linking monomer can also be used to promote bonding between the monomers constituting a photoresist polymer, thereby enhancing the yield of the photoresist polymer. For example, when 20 g of monomer is used in the polymerization reaction without using a cross-linker, about 4.8 g of a polymer having molecular weight of about 6,000 is obtained (yield: 24%). When the amount of the monomer is increased to 40 g, the amount of the polymer obtained is only about 6 g (i.e., the yield is abruptly lowered to about 15%). Thus, in order to prepare photoresist polymer in a large scale, it is desirable to use a cross-linking monomer to increase the yield and make production of the photoresist polymer commercially reasonable.